1. Field of the Invention
This present invention concerns the area of turbines, in particular that of the turbines of gas-turbine engines, and is aiming at a process for the manufacture by lost wax moulding of parts, in particular of blades, including cavities intended for the circulation of cooling fluids.
2. Description of the Related Art
The blades of turbines that are subjected to high thermal stresses include resources for cooling by the circulation of a cooling fluid, generally of air in the case of a gas-turbine engine, within cavities created within the blade.
Furthermore, the blades of turbines are now generally manufactured by the lost wax moulding process. This technique consists of creating a model in wax or another equivalent temporary material, of the part that one wishes to mould. The model is then placed in a ceramic shell mould. The latter is manufactured by successive dipping of the model in slurry mixtures containing a ceramic material and the addition of stucco to the layer formed between each dipping. The mould is dried, and then the wax that it contains is removed by a first oven treatment at a suitable temperature, followed by further oven treatment of the mould at high temperature in order to give it the strength necessary for the casting process. There then remains the replica of the model in the form of a mould into which the casting metal is poured. After cooling, the mould is broken to release the part. The latter then undergoes a simple finishing process.
When the blade includes cavities for the circulation of a cooling fluid, it is necessary to incorporate one or more cores into the model. This phase of the process includes firstly the separate manufacture of the core or cores by moulding them in a ceramic material consolidated by a binder, assembling them where appropriate, and then positioning of the core or cores in a wax mould. A model is thus moulded by the injection of wax into the wax mould, so that the model forms the replica of the part to be cast.
FIG. 1 shows, in section view perpendicular to the axis of the part, a wax mould 10 whose internal wall is the image of the part to be cast. Here, this mould is in two parts 10A and 10B. This mould includes a core 13. Here, the core is composed of a multiplicity of branches 13A to 13G parallel to each other and attached to a common root. Between them, the branches define spaces which will form partitions after pouring the metal. In order to provide for the retention of the core within the mould, small cones 15 are included in a plastic material. The function of these cones is to maintain a space between the walls of the core and the internal wall of the mould. When wax is injected, any irregularity of the layer caused by unwanted movement of the core is thus prevented.
The next stage consists of forming a ceramic mould around the model obtained after its extraction from the mould 10. The cones are no longer sufficient to guarantee the quality of the part, since they are removed with the wax at the moment of dewaxing the mould. In order to ensure correct positioning of the core in the shell mould, and to prevent it from moving at the moment of pouring the molten metal, rods are fitted on the model as illustrated in FIG. 2. This figure shows the wax model within the shell mould 12 in ceramic material. Here, these rods 16 are placed in the area close to the trailing edge, through the layer of wax 17, bearing against one face of the core 13. These rods are in platinum and are melted at the moment of pouring the molten alloy. Materials other than platinum, and performing the same function, can also be used.
To hold the rods 16 in position during the operations for creation of the shell mould, wax beads 18 are added around the rods. These beads are embedded in the wall of the shell mould 12 and the external part of the rods 16.
When the wax has been removed from the mould, the casting metal is poured, filling the space between the walls of the mould and those of the core. The rods are dissolved in the metal. After appropriate cooling, the elements constituting the core, for the creation of one or more cavities, are removed.
The result is the part shown in FIG. 3. During the finishing stage, it is necessary to remove the beads 18′ which have formed at the surface of the blade. When this operation is manual, it cannot be fully controlled, and this may give rise to significantly uneven wall thicknesses and lead to a high rejection rate. The particularly close tolerances of the wall thicknesses in these areas leave very little margin for error.
It can also be seen that the process does not allow one to obtain a part whose quality is optimal. The critical area of the blade is neither located perfectly in its environment nor immobilised during the injection of the wax, so that a positioning defect can already be observed at this stage of manufacture. This, added to the loss of material during the finishing operation, increases the risk that one will not achieve a product of satisfactory quality.
Moreover, the beads, being anchored in the ceramic shell, are the seat of mechanical stresses during removal of the metal, which can generate re-crystallised grains during the heat treatment process. The part then has to be scrapped.
It can also be observed that the operation for positioning the platinum rods and beads on the wax model is lengthy and difficult.
Finally, the operations of grinding and elimination of the beads have a cost which it is desirable to avoid.